What happens if you vary the size of the block (bs=512) or the count (count=2)? It could just be that the block size you specify doesn't match with the block size of the USB transfer and/or the SCSI/atapicam transfer and/or the block size of the HD.

In the various gmirror how-tos, it specifies bs=512 and count=10, which overwrites the first 5 KB of the disk. Lots more than the MBR and boot blocks.

However, you don't really need to zero those out to add a new disk to a system. Just slice it (and/or partition it), and format it as needed

I haven't tried that yet. It would take me two days to get to that adapter again, but yes I'll try your suggestions. I'll post updates as soon as I get to it.